Residents of Harvey are being urged to evacuate immediately as a wind change forces flames south towards the small South West town.

A bushfire emergency warning was extended on Thursday to cover Harvey, 140 kilometres south of Perth, after it was earlier issued for the nearby towns of Waroona and Preston Beach, where many residents have been forced to flee from the fast-moving blaze.

People in Harvey and surrounding areas are being advised by the Department of Fire and Emergency Services (DFES) to leave in a westerly direction along Uduc and Forestry roads, then south along Forrest Highway.

Meanwhile, more than 100 Waroona residents in an evacuation centre in Pinjarra have been told to find emergency accommodation in town, as it will be three to five days before they can return to their homes.

DFES Incident Controller Greg Mair said the South Western Highway could be closed for a few weeks because a wooden bridge at Samson Brook had sustained severe damage.

Mr Mair said the Forrest Highway, the other main route south, also remained closed.

Preston Beach remains cut off, but residents are being evacuated by boat.

Mr Mair said people in Harvey are being urged to leave, but there would be no forced evacuations.

The warning also applies to the Alcoa mine site and nearby properties.

Mr Mair said the Alcoa refinery was under threat and being actively defended.

In a statement, Alcoa said the fire has moved to the west of its Wagerup refinery and all efforts are being made to protect infrastructure.

The company said its Willowdale mine remains completely evacuated and will remain closed until the fire threat passed.

"There have been no reports of injuries to employees or contractors. The welfare of our employees is paramount and everything is being done to ensure employees in the refinery are safe," the statement said.

Meanwhile, a man in his 20's was taken to Peel Health Campus after suffering burns to his upper torso.

Nine patients have also been evacuated from Harvey Hospital as the town remains under threat.

The fire front is moving fast, in a westerly direction, and an emergency warning remains in place for the area bounded by Willowdale Road, Johnston Road, Forrest Highway, Dorsett Road, Williamson Road, Mayfield Road and Nanga Brook Road, including the Waroona town site, in the Shire of Waroona.

People west of the South Western Highway are being told to leave in a westerly direction if it is safe to do so.

Aerial water bombers, at least 20 firefighting trucks from the Department of Parks and Wildlife (DPAW) and 40 appliances from the Department of Fire and Emergency Services (DFES) are being used to fight the blaze.

Waroona is home to about 2,500 people.

DPAW spokesman Benson Todd said the fire was first reported at 7.25 on Wednesday, and it was started by a lightning strikeat Lane Pool Reserve near Dwellingup, 50 kilometres north-east of Waroona.

He said strong winds of 40 to 60 kilometres an hour had fanned the fire and pushed it towards Waroona.

Mr Benson said the decision to evacuate the towns was taken when embers started dropping in the streets.

He said there had been some unconfirmed reports that the fire has destroyed some buildings, but authorities had not been able to verify them at this stage.

There is a watch and act warning in place for people in an area bounded by Johnston Road, Willowdale Road, Forrest Highway, Riverdale Road and Logue Brook Dam and Clark Road to Nanga Road.